Centrifugal fans are well known to those skilled in the art. The use of a centrifugal fan by utility companies in connection with supply of air to boilers is not effectively competitive with axial flow fans. While a centrifugal fan has a higher efficiency at the maximum boiler load, the efficiency falls off drastically, as compared with an axial flow fan, due to the fact that flow control with a centrifugal fan is attained by adjustment of dampers. In an axial flow fan, adjustment of load is attained by varying the pitch of blades. In a typical installation, with each of a centrifugal fan and an axial fan operating at 75% boiler load, the efficiency of the centrifugal fan would be approximately 80%.
The present invention is directed to apparatus and method for enabling a centrifugal fan having dampers to operate at a higher efficiency as compared with an axial fan having variable pitch blades.